Financial Risk Management and Trading Activities - Fair Values for Corporation's Net Physical Derivative and Financial Assets and (Liabilities) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Collateral and counterparty netting (Assets)	$ (57)	$ (206)
Net derivative contracts (Assets)	331	975
Other assets measured at fair value on a recurring basis	33	0
Total assets measured at fair value on a recurring basis	364	975
Collateral and counterparty netting (Liabilities)	57	206
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(261)	(973)
Other liabilities measured at fair value on a recurring basis	(31)	(31)
Total liabilities measured at fair value on a recurring basis	(292)	(1,004)
Other fair value measurement disclosures Long-term debt	(7,307)	(6,641)
Commodity [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Assets)	309	1,140
Net derivative contracts (Assets)	309	1,140
Total derivative contracts (Liabilities)	(312)	(1,177)
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(312)	(1,177)
Interest Rate and Other [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Assets)	79	41
Net derivative contracts (Assets)	79	41
Other [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Liabilities)	(6)	(2)
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(6)	(2)
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Collateral and counterparty netting (Assets)	(53)	(15)
Net derivative contracts (Assets)	162	241
Other assets measured at fair value on a recurring basis	33	0
Total assets measured at fair value on a recurring basis	195	241
Collateral and counterparty netting (Liabilities)	53	15
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(175)	(82)
Other liabilities measured at fair value on a recurring basis	(31)	(31)
Total liabilities measured at fair value on a recurring basis	(206)	(113)
Level 1 [Member] | Commodity [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Assets)	210	254
Total derivative contracts (Liabilities)	(226)	(97)
Level 1 [Member] | Interest Rate and Other [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Assets)	5	2
Level 1 [Member] | Other [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Liabilities)	(2)
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Collateral and counterparty netting (Assets)	(4)	(191)
Net derivative contracts (Assets)	260	425
Other assets measured at fair value on a recurring basis	0	0
Total assets measured at fair value on a recurring basis	260	425
Collateral and counterparty netting (Liabilities)	4	191
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(342)	(883)
Total liabilities measured at fair value on a recurring basis	(342)	(883)
Other fair value measurement disclosures Long-term debt	(7,307)	(6,641)
Level 2 [Member] | Commodity [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Assets)	187	579
Total derivative contracts (Liabilities)	(346)	(1,071)
Level 2 [Member] | Interest Rate and Other [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Assets)	77	37
Level 2 [Member] | Other [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Liabilities)		(3)
Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Net derivative contracts (Assets)	120	497
Other assets measured at fair value on a recurring basis	0	0
Total assets measured at fair value on a recurring basis	120	497
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(20)	(285)
Total liabilities measured at fair value on a recurring basis	(20)	(285)
Level 3 [Member] | Commodity [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Assets)	120	494
Total derivative contracts (Liabilities)	(15)	(285)
Level 3 [Member] | Interest Rate and Other [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Assets)		3
Level 3 [Member] | Other [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Liabilities)	(5)
Counterparty netting [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Net derivative contracts (Assets)	(178)	(109)
Other assets measured at fair value on a recurring basis	0	0
Total assets measured at fair value on a recurring basis	(178)	(109)
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	178	109
Total liabilities measured at fair value on a recurring basis	178	109
Counterparty netting [Member] | Commodity [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Assets)	(177)	(108)
Total derivative contracts (Liabilities)	177	108
Counterparty netting [Member] | Interest Rate and Other [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Assets)	(1)	(1)
Counterparty netting [Member] | Other [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Liabilities)	1	1
Collateral [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Net derivative contracts (Assets)	(33)	(79)
Other assets measured at fair value on a recurring basis	0	0
Total assets measured at fair value on a recurring basis	(33)	(79)
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	98	168
Total liabilities measured at fair value on a recurring basis	98	168
Collateral [Member] | Commodity [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Assets)	(31)	(79)
Total derivative contracts (Liabilities)	98	168
Collateral [Member] | Interest Rate and Other [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total derivative contracts (Assets)	$ (2)